A conventional electric power steering system (EPS) assists a steering operation of a driver by driving force of a motor.
An EPS disclosed in JP 2011-176998A (US 2011/0018374 A1) uses a motor drive apparatus, in which an electronic control unit is provided at one axial side of a motor in an axial direction of the motor. In this motor drive apparatus, a motor case housing the motor therein has a hole provided at axial one end thereof, which is on the control unit side, and a column is inserted from the motor side. A bolt is inserted into a thread hole provided in a heat sink, which forms the control unit, and thread-engaged with the column. Thus the heat sink is attached to the motor case. A rotation angle sensor is mounted on a control unit substrate attached to the motor side of the heat sink. The rotation angle sensor detects the magnetic field of a magnet attached to an axial end of a shaft of the motor and outputs its detection signal to a control circuit provided on the control unit substrate. The control circuit detects a rotational angle of a rotor based on the detection signal of the rotation angle sensor and controls driving of the motor.
According to the EPS described above, however, it is likely that the rotation angle sensor mounted on the control unit substrate deviates in position from the rotation axis of the shaft because of a gap between the hole of the motor case and the column or a gap between the thread hole of the heat sink and the bolt. When the relative position between the rotation angle sensor and the magnet deviates, the control circuit will possibly not be able to detect the rotation angle of the rotor accurately from the detection signal of the rotation angle sensor.